There is a document summarization technique that automatically converts an input document into a short concise document. A typical document summarization technique extracts key sentences from the input document and contracts each of the key sentences thus extracted to generate a summary.
Examples of a method of contracting a sentence include a known method of pruning any unnecessary part of a tree structure (dependency structure, for example) of an input sentence. For example, Japanese Laid-open Patent Publication No. 2010-140468 discloses a technique of generating a contracted sentence candidate by combining phrases included in an input sentence based on a dependency structure of the input sentence provided with morphological analysis and dependency parsing. This technique calculates a generation probability of each candidate based on an importance of a certain word obtained from a corpus and an adjacency probability between certain phrases, and outputs a summary candidate having the highest generation probability of a length that is within a pre-specified length.